Two-piece and wound golf balls are made by molding a cover about a core. The core of a wound golf ball is either a liquid filled or solid center around which elastic thread has been wound. The core of a two-piece golf ball is a solid ball of material around which a cover is molded. Cores, of both the wound and two-piece golf ball, typically measure 1.4 to 1.6 inches (3.6 to 4.1 cm.) in diameter. The cover is molded about the core to produce a golf ball with the minimum United States Golf Association (USGA) diameter of 1.68 inches (4.27 cm.). So-called "British" golf balls are made in the same manner except the core size is typically between 1.35 to 1.55 inches (3.43 to 3.94 cm.) and have an overall minimum diameter of 1.62 inches (4.11 cm.).
Balata and SURLYN.RTM. are generally used as cover material. SURLYN.RTM. was discovered in the middle 1960's by DuPont and has been adopted by the golf ball industry for use as a cover composition because of its good cut resistance and ready availability. SURLYN.RTM. is a trademark for a group of ionomeric resins. U.S. Pat. No. 3,264,272 dated Aug. 2, 1968 teaches a wide variety of ionomeric resins and U.S. Pat. No. 3,454,280 dated July 8, 1969 teaches the use of SURLYN.RTM. as a cover material for golf balls. U.S. Pat. No. 3,819,768 dated June 25, 1974 alleges that a blend of sodium and zinc SURLYN.RTM. produces a more durable cover on a golf ball than a single SURLYN.RTM.. These ionomeric resins are copolymers of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic groups being neutralized by metal ions. Commercially, these ionomeric resins are available only as copolymers of ethylene and methacrylic or acrylic acid, wherein the acid groups are neutralized by sodium, zinc or lithium ions. The commercially available isomeric resins differ primarily in the quantities of each of the ingredients (ethylene, unsaturated acid and metal ions) present.
Golf ball covers are molded about a core in one of two ways, either by compression molding two hollow, semi-circular cover halves about a core or by injection molding cover stock around core. The molding of two semi-circular cover halves around a core is conventional and is generally accomplished by means of a frame containing a plurality of paired ball cup molds. Each ball cup mold bears a dimple pattern which is implanted on the cover halves when the frame is closed and the cover halves are subjected to heat and pressure. The heat and pressure cause the cover halves to join and form a solid, dimpled cover.
The semi-circular cover halves are formed in a conventional molding operation using a mandrel and a cup shaped mold half. The SURLYN.RTM. is injected into a closed cavity formed between the mandrel and the cup shaped mold half. Once the half shell hardens, the closed cavity is opened and the semi-circular cover half removed.
Injection molding of golf ball covers is accomplished in a conventional manner by placing cores inside retractable pin ball cup molds and injecting cover stock around the core. Generally the ball cup molds are housed in a frame.
Prior to molding the cover of a golf ball by injection molding or forming the semi-circular cover halves, the pellets of SURLYN.RTM. along with other materials which make up the SURLYN.RTM. cover stock material are placed in a hopper which feeds a heated barrel having a screw therein. The screw forces the SURLYN.RTM. cover stock through the barrel while the heat softens the SURLYN.RTM. into a fluid state.
A problem faced by golf ball manufacturers is water contamination in the SURLYN.RTM. cover stock. SURLYN.RTM. is very hygroscopic. In order to prevent the SURLYN.RTM. from picking up water, SURLYN.RTM. is sold in special containers. Golf ball manufacturers are required to buy a special "dry" SURLYN.RTM. from the producer of SURLYN.RTM. in order to get an acceptable starting material.
These special containers do not always prevent the SURLYN.RTM. from hydrating. When the moisture level of the SURLYN.RTM. increases beyond 2,000 ppm, the melt flow becomes unstable and the dyes added to cover stock for purposes of coloring the golf ball yield poor results. The instability with the color is especially true with orange colored golf balls.